1. Field of the Invention
This invention relates to systems and methods for controlling diesel engine emissions, including, for example, oxides of nitrogen emissions, particulate matter emissions, and the like.
2. Description of Related Art
New emission limits call for major reductions in oxides of nitrogen (NOx) and particulate matter (PM) emissions from diesel engines. To achieve low NOx/PM emission levels, engine manufacturers have developed systems for exhaust gas recirculation (EGR), while exhaust after-treatment suppliers have developed catalyst-based diesel particulate filters (CB-DPFs).
Using catalyst-based diesel particulate filters has been found to reduce PM emissions below the stringent requirements of the new heavy-duty emissions standards proposed for the 2005 through 2010 time frame. However, engine-out NOx emissions are still six to eight times higher than the heavy-duty emission standards for model year 2007 and three to one-hundred times greater than the 2009 light-duty emission standards.
To achieve the targeted NOx emission levels, research is being conducted with various post-combustion devices and/or systems. One such device/system is a selective catalytic reduction (SCR) device/system, which uses urea or ammonia as reductant. While selective catalytic reduction systems have been used in stationary applications for several years, their use and experience record with the mobile fleet is limited.
Another post-combustion NOx emissions control device/system is the NOx adsorber system, also known as lean NOx trap (LNT) or NOx adsorber catalyst (NAC), which requires a reductant for regeneration. The NOx adsorber system generally performs three distinct functions. The first function is to convert nitric oxide (NO) to nitrogen dioxide (NO2), typically done using a precious metal oxidation catalyst. As part of the second function, the nitrogen dioxide may then be trapped in the second stage of the lean NOx trap. The third function is to reduce NO2 to diatomic nitrogen.
The NOx adsorber system is a base metal oxide NO2 trap that requires periodic regeneration by enriching the exhaust via supplemental hydrocarbon. However, because of the chemical similarity of sulfur dioxide (SO2) and NO2, the lean NOx trap also has a great affinity for trapping SO2. Sulfur dioxide is more stable on the trap, making it harder to release. The SO2 release (desulfurization) also requires higher temperatures, which are potentially harmful to the efficiency and durability of the lean NOx trap. These factors make the physical time required for desulfurization much greater, for example, 10 times or more, than the time required for NOx regeneration (which is also known as NOx removal from trap). For these reasons the lean NOx trap system requires extremely low sulfur fuel and delicate strategy work to remove any sulfur that has deposited on the trap without damaging the trap. One of the requirements for desulfurization of a lean NOx trap is to conduct the desulfurization at as low a temperature as possible. However, because the rate of desulfurization is directly related to temperature, as well as the reductant mass, the lower the temperature, the longer the desulfurization event.
The implications of a long desulfurization event are very significant to the overall efficiency and/or complexity of the system. FIG. 1 shows a related research-developed engine emission control system 100 for a diesel engine 150. As shown in FIG. 1, the diesel engine emission control system 100 includes a diesel particulate filter (DPF) 102 and a lean NOx trap (LNT) 104, one of each DPF/LNT system, arranged in a in-series configuration. The engine emission control system 100 also includes an oxidation catalyst 106.
The system 100 illustrated in FIG. 1 generally offers low cost and complexity. However, the engine emission control system of FIG. 1 has efficiency drawbacks in that NOx emissions cannot be trapped while the system is undergoing NOx regeneration or desulfurization. Studies have shown that the NOx regeneration event can consume up to 6% of total operation time of the diesel engine and that desulfurization can take several minutes, with the frequency of the event being directly related to fuel sulfur level. This reduces the potential overall efficiency of the system. Furthermore, because the related DPF/LNT configuration shown in FIG. 1 requires full flow regeneration, there is a large fuel consumption penalty associated with bringing the full flow to overall reducing conditions.
FIG. 2 schematically illustrates another related research-developed configuration of a LNT-DPF emission control system 200 for a diesel engine 250. As shown in FIG. 2, the diesel engine emission control system 200 has dual, full-size parallel DPF/LNT systems 210, 220. Each of the parallel DPF/LNT systems 210, 220 includes a diesel particulate filter (DPF) 212, 222 and a lean NOx trap (LNT) 214, 224, one of each DPF/LNT system, arranged in an in-series configuration. The engine emission control system 200 also includes an oxidation catalyst 206.
Using the diesel engine emission control system 200 with parallel DPF/LNT system configuration, one bank of the system can be regenerated or desulfurized while the other bank performs the trapping function. In addition, the emissions flow can be diverted using valves 216, 226 to create low flow conditions on the regenerating bank. This arrangement reduces the required supplemental fuel needed to create a reducing environment, which also reduces the incurred fuel economy penalty associated with regeneration or desulfurization.
One drawback with the diesel engine emission control system 200 is that it is physically very large. In addition, because the system requires two full size CB-DPF catalysts, the system can be very expensive. Further, the frequency between NOx, regeneration is usually on the order of one minute. If a desulfurization event is being conducted and takes several minutes to complete, the trapping LNT will become saturated and need to be regenerated. This would cause NOx to begin to break through, requiring regeneration to be performed in the full flow bank/leg.
For the system 200 to maintain efficiency while desulfurizing, a third leg would be needed to allow desulfurization and regeneration to be conducted on the first and second banks while the third is trapping. This configuration is not very practical for production purposes as the system would be extremely large, require complex controls, and would be prohibitively expensive.
This invention provides systems and methods for diesel engine emission control utilizing sulfur protection for lean NOx traps.
In an aspect of the invention, the system relies on post-combustion injection (in-cylinder or in-exhaust) to provide reductant to the CB-DPF to generate exotherms high enough to regenerate a CB-DPF when necessary.
In another aspect of the invention, the system relies on supplemental fuel injection (in-exhaust) across a carbon monoxide generating (COG) catalyst to provide exotherms high enough to desulfurize a Sulfur-trap (S-trap).
In a further aspect of the invention, the COG catalyst is used to partially oxidize the fuel provided for regeneration from the supplemental fuel injector (SFI) to create carbon monoxide (CO) and smaller hydrocarbons, while simultaneously removing oxygen from the exhaust emitted by the diesel engine.
In a further aspect of the invention, the system diverts a portion of the total exhaust to reduce the supplemental reductant required to achieve a stoichiometric or rich air-fuel ratio (AFR) condition at the inlet of an LNT or S-trap to facilitate its regeneration.
In an aspect of the invention, the system uses a sulfur trap to prevent sulfur poisoning of the LNT, resulting in reduced LNT regeneration frequency requirements, thus reducing the incurred fuel economy penalty.
In further embodiments, the system makes use of certain driving conditions to perform partial regenerations to minimize fuel economy impact and to maintain high efficiency levels (specifically deceleration regenerations take advantage of low-flow exhaust conditions).
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.